Add explanation for sieve of eratosthenes

.gitignore

 # Own:
 *.exe
+.idea
 
 # From https://raw.githubusercontent.com/github/gitignore/master/Haskell.gitignore
 dist

Main.hs

 import SieveOfEratosthenes
 
-main = putStrLn (show (take 10 (primes) ) )
+main = putStrLn ("First 9 prime numbers: " ++ (show (take 9 (primes) ) ) )

TheSieveOfEratosthenes.md

+# The Sieve of Eratosthenes
+
+1. Write down the successive "plurals": 2, 3, 4, ...
+2. Repeat:
+    1. Take the first number that is not circled or crossed out.
+    2. Circle it.
+    3. Cross out its proper multiples.
+3. What is left (i.e. the circled numbers) are the successive prime numbers.
+
+**Example**
+
+```
+ 2   3   4   5   6   7   8   9   10   11   12   13   14   15   16   17   18   19   20   21   22   23   24   25 ...
+(2)  3   x   5   x   7   x   9   xx   11   xx   13   xx   15   xx   17   xx   19   xx   21   xx   23   xx   25 ...
+(2) (3)      5       7       x        11        13        xx        17        19        xx        23        25 ...
+(2) (3)     (5)      7                11        13                  17        19                  23        xx ...
+...
+```